Quitting smoking is difficult enough without also having to worry about the weight one seems destined to gain. Katahn (The T-Factor Diet) says at least two-thirds of the people who quit smoking will gain between 10 and 12 pounds. Part of the problem: cigarette smoking really is an aid to weight management, because it reduces one's desire to eat by directly affecting body chemistry. So it's important, he says, to increase one's metabolic rate to compensate for nicotine's metabolic effects. He advises that whenever the urge to smoke occurs after quitting, it's important to get up and move around. Brief physical activity not only reduces the urge to smoke, but can provide the lift that otherwise might have been obtained from nicotine. And yet, increased physical activity may not be enough. Katahn recommends reducing fat and carbohydrate intake without making any radical changes in one's regular diet, because he believes undertaking a traditional weight-reduction diet at such a time can actually lead to weight gain. To identify fats and carbohydrates, he provides an extensive calorie/fat/carbohydrate/gram counter and reduced-calorie menus. He also advocates deep relaxation and meditation techniques that will help curb those ``I've just got to have a cigarette'' moments. (Nov.)